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PM shifting on gay marriage, says sister

Prime Minister Tony Abbott's openly gay sister insists his views on same sex marriage are changing but concedes he may never vote in favour of legalising it.

Christine Forster says Mr Abbott is looking at the issue "in a slightly different way", after being "comfortably opposed" to it throughout his adult life.

"He's increasingly aware of how important this issue is to members of his family, to me - his sister, to his daughters, and to the rest of the world," she told an Australian Marriage Equality forum in Sydney on Wednesday.

But she doesn't expect him ever to vote favour of marriage equality.

"His view, for whatever reason he holds it, is his view and he's entitled to his view," Ms Forster said.

"But he hears me, he hears us."

Earlier, Australian Workers Union boss Paul Howes told the forum that federal Labor was wrong to allow its MPs a free vote on gay marriage.

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He said same sex marriage was an issue of social justice, not individual conscience.

"Just like with racial discrimination, it is vital to understand that our past discrimination against people in same sex relationships is not some sort of fundamental starting point for humanity," Mr Howes said.

"I want tonight to urge my party to finally disown this phony notion that we should be affording equal respect to both sides of the gay marriage debate, as if it were some exquisitely balanced moral quandary that could never be unlocked by us mere mortals. It is not."

Mr Howes wants the party's 2014 national conference to change the rules so there isn't a conscience vote.

But both leadership candidates Bill Shorten and Anthony Albanese say while they personally support same sex marriage, it should be a conscience matter.